.TH std::ranges::uninitialized_value_construct_n 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::uninitialized_value_construct_n \- std::ranges::uninitialized_value_construct_n

.SH Synopsis
   Defined in header <memory>
   Call signature
   template< no-throw-forward-iterator I >

   requires std::default_initializable<std::iter_value_t<I>>              \fI(since C++20)\fP

   I uninitialized_value_construct_n( I first, std::iter_difference_t<I>
   n );

   Constructs n objects of type std::iter_value_t<I> in the uninitialized memory area
   starting at first by value-initialization, as if by
   for (; n-- > 0; ++first)
       ::new (static_cast<void*>(std::addressof(*first)))
           std::remove_reference_t<std::iter_reference_t<I>>();

   If an exception is thrown during the initialization, the objects already constructed
   are destroyed in an unspecified order.

   The function-like entities described on this page are niebloids, that is:

     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.

   In practice, they may be implemented as function objects, or with special compiler
   extensions.

.SH Parameters

   first - the beginning of the range of elements to initialize
   n     - the number of elements to construct

.SH Return value

   The end of the range of objects (i.e., ranges::next(first, n)).

.SH Complexity

   Linear in n.

.SH Exceptions

   The exception thrown on construction of the elements in the destination range, if
   any.

.SH Notes

   An implementation may improve the efficiency of the
   ranges::uninitialized_value_construct_n, e.g. by using ranges::fill_n, if the value
   type of the range is TrivialType and CopyAssignable.

.SH Possible implementation

   struct uninitialized_value_construct_n_fn
   {
       template<no-throw-forward-iterator I>
       requires std::default_initializable<std::iter_value_t<I>>
       I operator()(I first, std::iter_difference_t<I> n) const
       {
           using T = std::remove_reference_t<std::iter_reference_t<I>>;
           if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>)
               return ranges::fill_n(first, n, T());
           I rollback{first};
           try
           {
               for (; n-- > 0; ++first)
                   ::new (const_cast<void*>(static_cast<const volatile void*>
                       (std::addressof(*first)))) T();
               return first;
           }
           catch (...) // rollback: destroy constructed elements
           {
               for (; rollback != first; ++rollback)
                   ranges::destroy_at(std::addressof(*rollback));
               throw;
           }
       }
   };

   inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};

.SH Example


// Run this code

 #include <iostream>
 #include <memory>
 #include <string>

 int main()
 {
     struct S { std::string m{"█▓▒░ █▓▒░ █▓▒░ "}; };

     constexpr int n{4};
     alignas(alignof(S)) char out[n * sizeof(S)];

     try
     {
         auto first{reinterpret_cast<S*>(out)};
         auto last = std::ranges::uninitialized_value_construct_n(first, n);

         auto count{1};
         for (auto it{first}; it != last; ++it)
             std::cout << count++ << ' ' << it->m << '\\n';

         std::ranges::destroy(first, last);
     }
     catch (...)
     {
         std::cout << "Exception!\\n";
     }

     // Notice that for "trivial types" the uninitialized_value_construct_n
     // zero-initializes the given uninitialized memory area.
     int v[]{1, 2, 3, 4, 5, 6, 7, 8};
     std::cout << ' ';
     for (const int i : v)
         std::cout << i << ' ';
     std::cout << "\\n ";
     std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));
     for (const int i : v)
         std::cout << i << ' ';
     std::cout << '\\n';
 }

.SH Output:

 1 █▓▒░ █▓▒░ █▓▒░
 2 █▓▒░ █▓▒░ █▓▒░
 3 █▓▒░ █▓▒░ █▓▒░
 4 █▓▒░ █▓▒░ █▓▒░
 1 2 3 4 5 6 7 8
 0 0 0 0 0 0 0 0

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to              Behavior as published              Correct behavior
   LWG 3870 C++20      this algorithm might create objects on a const  kept disallowed
                       storage

.SH See also

                                             constructs objects by value-initialization
   ranges::uninitialized_value_construct     in an uninitialized area of memory,
   (C++20)                                   defined by a range
                                             (niebloid)
                                             constructs objects by
   ranges::uninitialized_default_construct   default-initialization in an uninitialized
   (C++20)                                   area of memory, defined by a range
                                             (niebloid)
                                             constructs objects by
   ranges::uninitialized_default_construct_n default-initialization in an uninitialized
   (C++20)                                   area of memory, defined by a start and
                                             count
                                             (niebloid)
                                             constructs objects by value-initialization
   uninitialized_value_construct_n           in an uninitialized area of memory,
   \fI(C++17)\fP                                   defined by a start and a count
                                             \fI(function template)\fP
